The primary function of a photoinitiator is to initiate polymerization when the photoinitiator is irradiated with e.g., ultraviolet (UV) radiation. There are two main types of photoinitiators that can be used to initiate polymerization in the monomer or prepolymer upon irradiation—radical photoinitiators and cationic photoinitiators.
The most frequently used cationic photoinitiators are either organic iodonium or sulfonium salts. The mechanism by which a cationic photoinitiator acts, when irradiated, is that it forms an excited state which then breaks down to release a radical cation. This radical cation reacts with the solvent, or another hydrogen atom donor, eventually generating a protonic acid. The active species that initiates the polymerization is the protonic acid.
There continues to be a need in the art for cationic polymerization photoinitiators, including novel sulfonium salts, that can advantageously be used in UV curable compositions. The present invention addresses this need.